Currently sunload sensors, either single- or dual-channel, are commercially available. Most are optoelectronic devices and use a silicon photodiode as a sensing element. A number of vehicle manufacturers have integrated into their products dual-channel sunload sensors in an attempt to provide improved temperature stability in the cabin, and comfort to the passengers. Dual-channel sunload sensors must provide not only a measurement of the total solar heat load entering the car through the windshield, but also an indication of which side of the car, the driver side or the passenger side, is subjected to more or less heat, so that cool air from the air conditioning system be preferentially distributed on the side that requires more.
Present dual-channel sensors do not however fully respond to the need that they at once provide a measurement of the total heat load entering the cabin, and of the relative distribution of this radiative heat load on the driver and on the passenger, especially when the sun is at an angular position that is low with respect to the horizontal plane of the vehicle. Typically, when the sun is low, the total heat load is under-estimated, and the comparative signal of the two channels becomes an unreliable indicator of the relative fractions of the heat load that are affecting the two sides of the cabin.
In addition, some present dual-channel sensors require the presence of sensing elements that are mounted at some angle from one another, a feature that renders their fabrication more complex and costly.